In a coal-gasification plant a pulverised carbonaceous fuel, such as coal, is transformed into a product gas consisting mainly of synthesis gas. The gasification plant typically comprises a gasification reactor, or gasifier, wherein the pulverised carbonaceous fuel is gasified under high pressure and high temperature conditions. The synthesis gas leaving the gasifier may carry along with it fly ash or fly slag, or ash-forming constituents which may consist of alkali metal chlorides, silicon and/or aluminium oxides. For the purpose of this specification, all such solid particulates entrained with the synthesis gas are referred to with the term fly ash.
In order to obtain a clear product gas, the unwanted fly ash can be separated from the gas stream on the high-pressure and high-temperature side of the gasification plant, then depressurised and cooled prior to disposal. A minor amount of residual synthesis gas can be carried by, entrained with or absorbed on the fly ash. To remedy this undesirable situation, the fly ash must be detoxified prior to disposal. In particular, residual synthesis gas must be stripped from the fly ash.
An apparatus and method for removal and disposal of fly ash from a high-temperature, high-pressure synthesis gas stream has been proposed in U.S. Pat. No. 4,838,898. This US patent describes fly ash being separated from a stream of synthesis gas coming from a gasifier at high temperature and high pressure. A batch of the fly ash is collected using a separator in the form of a cyclone separator. The batch is subsequently sluiced down to ambient pressure in a lock hopper. From the lock hopper, the batch is transported to a stripper vessel via connecting means in the form of a pneumatic conveyor line provided with a discharge valve.
The batch load of fly ash is held in the stripper vessel while a continuous flow of low-pressure nitrogen into the bottom of the stripper and up through the batch of fly ash in the vessel is maintained. The flow of nitrogen gas strips the synthesis gas from the fly ash with the gases being discharged through an open valve in a vent line from the top of the stripper vessel. The batch is treated this way in the stripper vessel until the content of carbon monoxide in the discharged gas has dropped to below a pre-determined value. Then the batch of fly ash is released for disposal.
In the above described apparatus and method, fresh fly ash must be collected into a new batch during the time that the prior batch is being stripped in the stripper vessel.
This poses a certain minimum amount of powder that has to be collected in each batch load. To this end, the above described apparatus and method make use of an intermediate accumulating capacity provided by an accumulator in the form of an intermediate collecting vessel and/or a lower part of the separator and/or a lock hopper, or a combination of those.